World’s deep mines have their own special features

It is customarily thought that the gold mines of South Africa’s Rand pioneered deep mining. Not so. That distinction was held for many years by the Kolar gold field in India.

While the deepest Rand mines were just beginning to nudge 7,000 ft., the Kolar mines had been operating at 8,000 ft. for many years. And when their heyday was over in the mid-1960s, they had reached a depth of more than 11,000 ft.

Kolar is in Karnataka state, southern India. It is scarcely known today yet it was once one of the world’s famous gold mining camps. It was also there that the first theories on the behavior of rock at depth were formulated. The field stretched more than 4.5 miles and developed five mines on the aptly named “Champion” lode. During its peak years it produced at the rate of 500,000 oz. per year. Today’s production is an insignificant 25,000 oz. per year and the greatest value the mines now have is as a research establishment — fittingly enough, for the study of rock mechanics.

The world’s deepest mine now is Anglo American’s Western Deep Levels on the western extension of the Transvaal’s Rand. (The mine produced 1.32 million oz. gold in 1991; Ontario’s three Hemlo mines produced 1.35 million oz. in 1990.)

With deep mining came the problem of rock stresses. For example, last month, four men were killed in a rockburst at WDL bringing to 21 the number of fatalities at this one mine alone in the current year.

This particular rockburst was triggered by an earth tremor, but the depth of a mine and the nature of the enclosing rocks are key factors in the occurrence of these destructive phenomena.

Where the ore and the overlying rocks are sedimentary and layered as they are on the Rand, there appears to be a greater propensity for intense bursting. When the location of the ore is controlled by faults, by the contacts of dissimilar volcanic/granitic rocks and discontinuities of other kinds, there is still bursting but it tends to be less intense and the damage more localized.

In simplified terms, the respective conditions may be likened to a railroad bridge built from flat concrete beams and one made up from innumerable, dimension-stone blocks. If an overloaded train should happen to cross, the bridges will respond differently.

In the one case, the concrete will accumulate the stresses and strains until it can absorb no more. It will then fracture violently, collapsing the structure. In the other case, the host of joints between the blocks of the stone bridge will act as safety valves for the stresses. The individual blocks will move ever so slightly and the stresses will have fewer opportunities to accumulate. Even where they should succeed in accumulating, their energy will be on a reduced scale and their release less destructive. Anglo American has described WDL as the world’s most seismically active mine. It is the deepest of the many deep mines of the West Rand, reaching to 12,730 ft. Planning is now on the books to go to 14,000 ft., the lowest level at which mining can be continued profitably. Temperature is the limiting factor. At 12,730 ft., the rock temperature registers 131F and the equivalent of 37,000 tons of ice per day is needed to keep the working temperature within bounds.

In comparison, North American mines can almost be characterized as pikers. The deepest is the now-closed Lake Shore (Kirkland Lake) at 8,150 ft. Its neighbor, Macassa, is 7,238 ft. and then there is Creighton (Sudbury) at 7,137 ft. (Creighton does have a lower working level at 7,200 ft. but access to that is by ramp).

In the U.S., Lucky Friday shaft (Idaho) was designed for 7,700 ft. but it is currently bottomed (for the foreseeable future) at 6,004 ft. One of the world’s oldest mines still operating is the Morro Velho in Brazil. It was started in 1784. Up-to-date information is lacking but that mine’s depth is believed to exceed 8,000 ft.